1. Field of the Invention
The present invention relates to a liquid crystal display substrate that forms a part of a liquid crystal display used in a display section of an information apparatus or the like, a liquid crystal display having the same and a method of manufacturing the same.
2. Description of the Related Art
In general, a liquid crystal display comprises two substrates having a transparent electrode and a liquid crystal sealed between the two substrates. The liquid crystal is driven by applying a voltage between the two transparent electrodes to control the transmittance of light through the liquid crystal, which allows a desired image to be displayed. An active matrix liquid crystal display is comprised of a TFT substrate having thin film transistors (TFTs) for switching respective pixels formed thereon and a common electrode substrate having a common electrode formed thereon. A recent increase in the need for liquid crystal displays has resulted in diverse requirements for liquid crystal displays. In particular, there are strong demands for improvements of viewing angle characteristics and display quality, and VA (vertically aligned) mode liquid crystal displays are regarded as promising means for satisfying such demands.
A VA mode liquid crystal display is comprised of two substrates which have been subjected to a vertically aligning process on surfaces thereof facing each other and a liquid crystal having negative dielectric anisotropy sealed between the two substrates. The liquid crystal molecules of the liquid crystal are characterized by homeotropic alignment and are aligned substantially perpendicularly to the substrate surfaces when no voltage is applied between the electrodes. They are aligned substantially in parallel with the substrate surfaces when a predetermined voltage is applied between the electrodes and are aligned at an angle to the substrate surfaces when a voltage lower than said voltage is applied.
MVA (multi-domain vertical alignment) type liquid crystal displays are recently attracting attention from the viewpoint of improvement of viewing angle characteristics of liquid crystal displays. In the case of an MVA type display, a pixel is divided into a plurality of domains using alignment regulating structures such as linear protrusions and slits provided on two substrates to achieve separate alignment in which liquid crystal molecules are tilted in a different direction in each domain.
FIG. 35 shows a configuration of an MVA type liquid crystal display and shows an arrangement of linear protrusion formed as alignment regulating structures on two substrates. FIG. 35 shows three pixels in red (R), green (G) and blue (B). As shown in FIG. 35, linear protrusions 104 are formed on a TFT substrate 108 and linear protrusions 106 are formed on a common electrode substrate 110. The linear protrusions 104 and 106 are formed at an angle to the pixels. Each of the R, G and B pixel regions is defined by a black matrix (BM) 102 formed on the common electrode substrate 110. The BM 102 serves as a light shield for a storage capacity bus line extending across each pixel substantially in the middle thereof and a storage capacity electrode located above the same (both of which are not shown).
FIG. 36 is a sectional view of the liquid crystal display taken along the line X—X in FIG. 35. As shown in FIG. 36, the TFT substrate 108 has a pixel electrode 114 formed for each pixel on a glass substrate 112. The figure omits an insulation film, drain bus lines, a protective film, and so on formed on the glass substrate 112. The linear protrusions 104 are formed on the pixel electrodes 114. A vertical alignment film 116 is formed to cover the pixel electrodes 114 and linear protrusions 104 entirely. The common electrode substrate 110 has the BM 102 formed on the glass substrate 112. Resin color filter(CF)layers R, G and B (FIG. 36 shows the filters G and B only) are formed in each of the pixel regions defined by the BM 102 on the glass substrate 112. A common electrode 118 is formed on the region CF layers R, G and B, and the linear protrusions 106 are formed on the common electrode 118. Further, a vertical alignment film 116 is formed to cover the common electrode 118 and linear protrusions 106 entirely. Spherical spacers 122 made of plastic or glass for maintaining a gap (cell gap) between the substrates 108 and 110 and a liquid crystal LC is sealed between the TFT substrate 108 and common electrode substrate 110.
FIG. 37 is a sectional view of the liquid crystal display taken along the line Y—Y in FIG. 35, and it shows a state of the liquid crystal LC when no voltage is applied. As shown in FIG. 37, liquid crystal molecules (represented by columns in the figure) are aligned substantially perpendicularly to the vertical alignment films 116 on the two substrates 108 and 110. Therefore, liquid crystal molecules in the regions where the linear protrusions 104 and 106 are formed are aligned substantially perpendicularly to the surface of the linear protrusions 104 and 106 and are aligned at a slight angle to the normal of the two substrates 108 and 110. Since polarizers (not shown) are provided in a crossed Nicols configuration outside the two substrates 108 and 110, black display is achieved when no voltage is applied.
FIG. 38 is a sectional view of the liquid crystal display taken along the line Y—Y in FIG. 35 similarly to FIG. 37, and it shows a state of the liquid crystal LC when a voltage is applied. The broken lines in the figure represent lines of electric force between the pixel electrodes 114 and common electrode 118. As shown in FIG. 38, when a voltage is applied between the pixel electrodes 114 and common electrode 118, the electric field is distorted in the vicinity of the linear protrusions 104 and 106 which are made of a dielectric material. As a result, the tilting angles of liquid crystal molecules having negative dielectric anisotropy are regulated, and the tilting angles can be controlled depending on the field intensity to display gray shades.
At this time, if the linear protrusions 104 and 106 are provided in linear configurations as shown in FIG. 35, liquid crystal molecules in the vicinity of the linear protrusions 104 and 106 are tilted in two directions which are orthogonal to the extending directions of the linear protrusions 104 and 106, the tilting directions being symmetrically defined about the linear protrusions 104 and 106. Since the liquid crystal molecules in the vicinity of the linear protrusions 104 and 106 are at a slight angle to a direction perpendicular to the two substrates 108 and 110 even when no voltage is applied, they are quickly tilted in response to the field intensity. The tilting directions of liquid crystal molecules in the neighborhood are sequentially determined in accordance with the behavior of the above-mentioned liquid crystal molecules, and the tilting angles depend on the field intensity. As a result, alignment separation is achieved at the linear protrusions 104 and 106.
FIG. 39 is a sectional view taken along a line Y—Y of a liquid crystal display as shown in FIG. 35 in which slits 120 are formed in place of the linear protrusions 104, the figure showing a state of the display when no voltage is applied. As shown in FIG. 39, the slits 120 which are alignment regulating structures are formed by removing the pixel electrodes 114. Liquid crystal molecules are aligned substantially perpendicularly to the vertical alignment films 116 on the two substrates 108 and 110 similarly to the liquid crystal molecules shown in FIG. 37.
FIG. 40 is a sectional view of the liquid crystal display taken along the line Y—Y similarly to FIG. 39, and it shows a state of a liquid crystal LC when a voltage is applied. As shown in FIG. 40, lines of electric force substantially similar to those in the regions where the linear protrusions 104 are formed as shown in FIG. 38 are formed in the regions where the slits 120 are formed. As a result, alignment separation is achieved about the linear protrusions 106 and slits 120. FIGS. 37 and 40 omit the spherical spacers 122 for maintaining a cell gap.
FIG. 41 is a sectional view of the liquid crystal display taken along the line Z—Z in FIG. 35 showing the neighborhood of a drain bus line 126. As shown in FIG. 41, the TFT substrate 108 has an insulation film 124 covering an entire surface of the glass substrate 112. The drain bus line 126 is formed on the insulation film 124. A protective film 128 is formed on the entire surface of the drain bus line 126. A pixel electrode 114 for each pixel is formed on the protective film 128. A black matrix BM 102 is formed on a common electrode substrate 110 provided in a face-to-face relationship with the TFT substrate 108 such that it serves as a light shield for regions on the TFT substrate 108 where no pixel electrode 114 is formed (edges of pixel regions).
The conventional MVA type liquid crystal display has the problem of darkness of display because of low transmittance of the panel. The low panel transmittance is attributable to various factors including a reduction in the numerical aperture caused by misalignment between the TFT substrate 108 and common electrode substrate 110, a reduction in the numerical aperture attributable to the alignment regulating structures (the linear protrusions 104 and 106 or slits 120), and irregularities in the alignment of the liquid crystal in the vicinity of the spherical spacers 122.
Because of significantly improved viewing angle characteristics, MVA type liquid crystal displays are preferably used as monitors for personal computers and the like for which high luminance has relatively low importance. However, in order to use them as display sections of DVD (digital versatile disk) players or televisions for which high luminance is an important requirement, it is necessary to provide a brighter back-light or to use a special sheet for aligning light-emitting directions to improve luminance in a particular direction. This has resulted in the problem of an increase in the manufacturing cost.
Further, the formation of linear protrusions, an insulation layer, and so on as alignment regulating structures increases manufacturing steps when compared to manufacturing steps for normal substrates, which also results in an increase in the manufacturing cost.